1. Field of the Invention
This invention relates to fail safe optical devices, and more particularly to active fail safe optical devices which advantageously employ electro-absorptive devices to significantly attenuate light in a through path.
2. Pior Art
One of the basic building blocks of a conventional multi-drop optical bus system (whether a T-Bus or ring topology) is the T-Coupler. Light traveling along the bus line is diverted at a T-Coupler to a peripheral optically responsive device, data generated in such a device and the light returned to the bus line by use of electronics associated with the peripheral device.
Two types of T-Couplers, active or passive can be used. Passive T's ensure that light diverted to a peripheral unit will be returned to the bus line without reliance on the electronics of the peripheral units. Thus, truly passive T's have the advantage of being inherently fail safe. However, the number of hookups to the bus line is usually limited to usually under ten peripheral units (or terminals) due to the coupling loss associated with each such T-Coupler.
Active T's divert light in a bus line to optically responsive devices which rely on opto-electronics to ensure the return of diverted light to the bus line. The electronics of an active T can be used to compensate for the inevitable transmission losses by regenerating light coming into the T-Coupler. Such a regenerative device is called a repeater. Regeneration has the advantage of allowing virtually unlimited bus length except for systems considerations such as delay time introduced by the electronics and opto-electronics transducers.
Obviously a major disadvantage for the active T's is their dependence on electronics to ensure the continued propogation of light along the bus line and thus to other terminals farther down the line.
Active T's can be made fail safe by providing optical through paths from the input port to the output port of an active T that allow at least a fraction of the light on the bus line to continue along the bus line in the event of failure of the active T's electronics. Through paths can be formed by using a power divider inserted after light has been returned to bus line as shown in FIG. 1. This technique has a disadvantage that the power divider can produce back scattered light (as shown by the dashed line of FIG. 1) which in turn can seriously limit the total receiver to transmitter gain when the receiver and transmitter are simultaneously operated.
A fail safe optical T-Coupler is disclosed in U.S. Pat. No. 4,246,475 to Altman. Therein a passive, non amplifying light path is connected in parallel with a light energy amplifying means. The passive light path transmits light to the output of the T-Coupler at all times. In order to avoid undesirable destructive interference between light signals passing through the passive path and light signals propogating in the amplifying path, it is necessary that the passive path have an optical length that impresses a delay on light signals passing therethrough which is substantially equal to the optical delay imposed by transmission of light through the amplifying path.
Another T-Coupler is disclosed in U.S. Pat. No. 4,166,946 to Chown et al. Therein a by-pass arrangement is proposed at various terminal sites on a bus line. The by-pass consists of an optical fiber which allows some bus line light to pass the terminal site at all times. If the terminals are repeaters, the passive by-pass fibers are designed to have a considerable amount of attentuation. For non repeating terminals, most of the optical power is coupled into the by-pass. For repeating terminals, problems due to the delay difference between light signals in the by-pass fiber and light regenerated at a repeater terminal are addressed by suggesting that optical delay could be added to the by-pass line or by designing the by-pass fiber to further attentuate the by-pass signal. For non-repeating terminals, Chown assumed that the optical signals transmitted by each terminal site are sufficiently high and the distribution of repeaters along the bus line is sufficiently frequent to allow the original signals (i.e. the non by-pass signals) to swamp any by-pass signal at any particular location on the bus line.
In both Chown and Altman, it is clear that the through path (i.e. Altman's passive, non amplifying light path and Chown's by-pass fiber) is subject to severe limitations imposed by the requirement of matching the optical delay in the through path with the optical delay in the peripheral unit coupled to the T-Coupler. Further, Chown discloses attentuation of the light signal in the through path only by selecting optic fibers which inherently attentuate light signals. Thus, in Chown, the same attentuated signal will exit the through path at repeater terminals when the electronics of the repeater terminal fails as will exit the through path when the electronics of the repeater terminal does not fail. Also, Chown and Altman do not disclose devices integrable with an electro-absorptive substrate.